Control unit of induction heating unit, induction heating system, and method of controlling induction heating unit

ABSTRACT

A control unit of an induction heating unit controls AC power output to a heating coil of a transverse type induction heating unit that allows an alternating magnetic field to intersect a sheet surface of a conductive sheet that is being conveyed to inductively heat the conductive sheet. The control unit includes: a magnetic energy recovery switch that outputs AC power to the heating coil; a frequency setting unit that sets an output frequency in response to at least one of the relative permeability, resistivity, and sheet thickness of the conductive sheet; and a gate control unit that controls a switching operation of the magnetic energy recovery switch on the basis of the output frequency set by the frequency setting unit.

FIELD OF THE INVENTION

The present invention relates to a control unit of an induction heatingunit, an induction heating system, and a method of controlling theinduction heating unit. Particularly, the present invention is suitablefor being used to make an alternating magnetic field intersect aconductive sheet in a substantially orthogonal manner so as toinductively heat the conductive sheet.

Priority is claimed on Japanese Patent Application No. 2009-283255,filed Dec. 14, 2009, the content of which is incorporated herein byreference.

DESCRIPTION OF RELATED ART

In the conventional techniques, for example, an induction heating unithas been used when heating a conductive sheet such as a steel sheet thatis conveyed through a manufacturing line. The induction heating unit isprovided with a heating coil, and heats the conductive sheet using aneddy current induced by the heating coil. In this induction heatingunit, the eddy current is caused to the conductive sheet by analternating magnetic field (AC magnetic field) generated by the heatingcoil, Joule heat is generated in the conductive sheet due to the eddycurrent. As an example of the induction heating unit, a transverse typeinduction heating unit is disclosed. In the transverse type inductionheating unit, the alternating magnetic field is applied to theconductive sheet in a manner that intersects a sheet surface of theconductive sheet, which is an object to be heated, to be substantiallyorthogonal thereto.

As a method of controlling the transverse type induction heating unit, atechnique disclosed in Patent Citation 1 may be exemplified. In PatentCitation 1, a capacitor is provided in parallel to the heating coil thatmakes up the induction heating unit, the heating coil and the capacitormake up a parallel resonance circuit, and power is supplied to theheating coil by a parallel resonance type inverter.

PATENT CITATION

[Patent Citation 1] Japanese Unexamined Patent Application, FirstPublication No. 2002-313547

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, when the heating coil of the induction heating unit is seenfrom a power supply unit (power supply circuit) of the induction heatingunit, the inductance varies in response to the sheet conveyance speed ofthe conductive sheet that is an object to be heated by the inductionheating unit (in the following description, this inductance is referredto as apparent inductance as necessary). Specifically, when the sheetconveyance speed of the conductive sheet becomes fast (or slow), theapparent inductance becomes small (or large).

However, in the technique disclosed in Patent Citation 1, the heatingcoil and the capacitor make up the parallel resonance circuit.Therefore, when the apparent inductance varies, the power frequency,which is supplied to the heating coil, also varies. For example, whenthe sheet conveyance speed of the conductive sheet becomes fast andthereby the apparent inductance becomes small, the frequency of thepower supplied to the heating coil increases. In this manner, when thefrequency of the power supplied to the heating coil increases, thetemperature in the vicinity of an end portion (edge) of the conductivesheet in the sheet width direction becomes higher than that in thevicinity of the central portion of the conductive sheet in the sheetwidth direction. Therefore, there is a concern in that a temperaturedistribution of the conductive sheet in the sheet width direction may benon-uniform.

As described above, in the conventional techniques, in a case where theconductive sheet is heated by using the transverse type inductionheating unit, there is a problem in that as the sheet conveyance speedof the conductive sheet varies, the temperature distribution of theconductive sheet in the sheet width direction becomes non-uniform.

The present invention has been made in consideration of this problem,and an object of the present invention is to realize a temperaturedistribution that is more uniform than that in the conventionaltechniques by preventing the temperature distribution of the conductivesheet in the sheet width direction from being non-uniform even when thesheet conveyance speed of the conductive sheet varies in a case wherethe conductive sheet is heated using a transverse type induction heatingunit.

Methods for Solving the Problem

(1) A control unit of an induction heating unit according to an aspectof the present invention controls AC power output to a heating coil of atransverse type induction heating unit allowing an alternating magneticfield to intersect a sheet surface of a conductive sheet that is beingconveyed to inductively heat the conductive sheet. The control unitincludes: a magnetic energy recovery switch that outputs AC power to theheating coil, a frequency setting unit that sets the output frequency inresponse to at least one of the relative permeability, resistivity, andsheet thickness of the conductive sheet; and a gate control unit thatcontrols a switching operation of the magnetic energy recovery switch onthe basis of the output frequency set by the frequency setting unit.

(2) In the control unit of an induction heating unit according to (1),the frequency setting unit may acquire attribute information thatspecifies the relative permeability, resistivity, and sheet thickness ofthe conductive sheet, and may select a frequency corresponding to theacquired attribute information as the output frequency with reference toa table in which the relative permeability, resistivity, and sheetthickness of the conductive sheet, and the frequency are correlated witheach other and are registered in advance.

(3) The control unit of an induction heating unit according to (1) or(2) may further include: an output current setting unit that sets anoutput current value in response to at least one of the relativepermeability, resistivity, and sheet thickness of the conductive sheet;a current measuring unit that measures an alternating current that flowsto the induction heating unit; and a power supply unit that supplies DCpower to the magnetic energy recovery switch and adjusts an alternatingcurrent that is measured by the current measuring unit to the outputcurrent value that is set by the output current setting unit, whereinthe magnetic energy recovery switch may be supplied with the DC power bythe power supply unit and may output the AC power to the heating coil.

(4) In the control unit of an induction heating unit according to (3),the output current setting unit may acquire attribute information thatspecifies the relative permeability, resistivity, and sheet thickness ofthe conductive sheet, and may select a current value corresponding tothe acquired attribute information as the output current value withreference to a table in which the relative permeability, resistivity,and sheet thickness of the conductive sheet, and the current value arecorrelated with each other and are registered in advance.

(5) The control unit of an induction heating unit according to (1) or(2) may further include an output transformer that is disposed betweenthe magnetic energy recovery switch and the induction heating unit,lowers the AC voltage that is output from the magnetic energy recoveryswitch, and outputs the lowered AC voltage to the heating coil.

(6) In the control unit of an induction heating unit according to (1) or(2), the magnetic energy recovery switch may include first and second ACterminals that are connected to one end and the other end of the heatingcoil, respectively, first and second DC terminals that are connected toan output terminal of the power supply unit, a first reverseconductivity type semiconductor switch that is connected between thefirst AC terminal and the first DC terminal, a second reverseconductivity type semiconductor switch that is connected between thefirst AC terminal and the second DC terminal, a third reverseconductivity type semiconductor switch that is connected between thesecond AC terminal and the second DC terminal, a fourth reverseconductivity type semiconductor switch that is connected between thesecond AC terminal and the first DC terminal, and a capacitor that isconnected between the first and second DC terminals, the first reverseconductivity type semiconductor switch and the fourth reverseconductivity type semiconductor switch may be connected in series insuch a manner that conduction directions at the time of a switch-offbecome opposite to each other, the second reverse conductivity typesemiconductor switch and the third reverse conductivity typesemiconductor switch may be connected in series in such a manner thatconduction directions at the time of the switch-off become opposite toeach other, the first reverse conductivity type semiconductor switch andthe third reverse conductivity type semiconductor switch may have thesame conduction direction at the time of the switch-off as each other,the second reverse conductivity type semiconductor switch and the fourthreverse conductivity type semiconductor switch may have the sameconduction direction at the time of the switch-off as each other, andthe gate control unit may control a switching operation time of thefirst and third reverse conductivity type semiconductor switches and aswitching operation time of the second and fourth reverse conductivitytype semiconductor switches on the basis of the output frequency that isset by the frequency setting unit.

(7) An induction heating system according to another aspect of thepresent invention allows an alternating magnetic field to intersect asheet surface of a conductive sheet that is being conveyed toinductively heat the conductive sheet. The induction heating systemincludes: the control unit of an induction heating unit according to (1)or (2); a heating coil that is disposed to face the sheet surface of theconductive sheet; a core around which the heating coil is wound; and ashielding plate which is disposed to face a region including an edge ofthe conductive sheet in the width direction and is formed from aconductor having a relative permeability of 1.

(8) In the induction heating system according to (7), the shieldingplate may have a depressed portion.

(9) In the induction heating system according to (8), the shieldingplate may be disposed in such a manner that a region, which is closer tothe edge of the conductive sheet than a region in which an eddy currentflowing to the conductive sheet becomes the maximum, and the depressedportion face each other.

(10) A method of controlling an induction heating unit according tostill another aspect of the present invention controls AC power, whichis output to a heating coil of a transverse type induction heating unitallowing an alternating magnetic field to intersect a sheet surface of aconductive sheet that is being conveyed to inductively heat theconductive sheet. The method includes: outputting AC power to theheating coil by a magnetic energy recovery switch; setting an outputfrequency in response to at least one of a relative permeability,resistivity, and sheet thickness of the conductive sheet; andcontrolling a switching operation of the magnetic energy recovery switchon the basis of the output frequency that is set.

(11) In the method of controlling an induction heating unit according to(10), the output frequency may be set by acquiring attribute informationthat specifies the relative permeability, resistivity, and sheetthickness of the conductive sheet, and by selecting a frequencycorresponding to the acquired attribute information as the outputfrequency with reference to a table in which the relative permeability,resistivity, and sheet thickness of the conductive sheet, and thefrequency are correlated with each other and are registered in advance.

(12) The method of controlling an induction heating unit according to(10) or (11) may further include: setting an output current value inresponse to at least one of the relative permeability, resistivity, andsheet thickness of the conductive sheet; measuring an alternatingcurrent that flows to the induction heating unit; and supplying DCpower, which is necessary for adjusting an alternating current that ismeasured to the output current value that is set, to the magnetic energyrecovery switch.

(13) In the method of controlling an induction heating unit according to(12), the output current value may be set by acquiring attributeinformation that specifies the relative permeability, resistivity, andsheet thickness of the conductive sheet, and by selecting a currentvalue corresponding to the acquired attribute information as the outputcurrent value with reference to a table in which the relativepermeability, resistivity, and sheet thickness of the conductive sheet,and the current value are correlated with each other and are registeredin advance.

(14) In the method of controlling an induction heating unit according to(10) or (11), an AC voltage that is output from the magnetic energyrecovery switch may be lowered by an output transformer, and the loweredAC voltage may be output to the heating coil.

(15) In the method of controlling an induction heating unit according to(10) or (11), the magnetic energy recovery switch may include first andsecond AC terminals that are connected to one end and the other end ofthe heating coil, respectively, first and second DC terminals that areconnected to an output terminal of the power supply unit, a firstreverse conductivity type semiconductor switch that is connected betweenthe first AC terminal and the first DC terminal, a second reverseconductivity type semiconductor switch that is connected between thefirst AC terminal and the second DC terminal, a third reverseconductivity type semiconductor switch that is connected between thesecond AC terminal and the second DC terminal, a fourth reverseconductivity type semiconductor switch that is connected between thesecond AC terminal and the first DC terminal, and a capacitor that isconnected between the first and second DC terminals, the first reverseconductivity type semiconductor switch and the fourth reverseconductivity type semiconductor switch may be connected in series insuch a manner that conduction directions at the time of a switch-offbecome opposite to each other, the second reverse conductivity typesemiconductor switch and the third reverse conductivity typesemiconductor switch may be connected in series in such a manner thatconduction directions at the time of the switch-off become opposite toeach other, the first reverse conductivity type semiconductor switch andthe third reverse conductivity type semiconductor switch may have thesame conduction direction at the time of the switch-off as each other,the second reverse conductivity type semiconductor switch and the fourthreverse conductivity type semiconductor switch may have the sameconduction direction at the time of the switch-off as each other, andthe AC power may be output to the heating coil by controlling aswitching operation time of the first and third reverse conductivitytype semiconductor switches and a switching operation time of the secondand fourth reverse conductivity type semiconductor switches on the basisof the output frequency that is set.

Effects of the Invention

According to the control unit of an induction heating unit according tothe aspect of the present invention, the switching operation of themagnetic energy recovery switch is controlled on the basis of thefrequency in response to at least one of the relative permeability,resistivity, and sheet thickness of the conductive sheet that is beingconveyed, and the AC power of this frequency is output from the magneticenergy recovery switch. Therefore, the AC power of the frequencycorresponding to the attribute of the conductive sheet that is beingconveyed can be applied to the heating coil without being subjected to arestriction in regard to an operation with a resonant frequency.Therefore, it is possible to prevent the temperature distribution of theconductive sheet in the sheet width direction from being non-uniformeven when a sheet conveyance speed of the conductive sheet varies in acase where the conductive sheet is heated using a transverse typeinduction heating unit. In addition, the AC power with the frequency inresponse to the attribute of the conductive sheet that is being conveyedcan be supplied to the heating coil independently from operationalconditions, such that the induction heating control can be performed ina relatively simple and reliable manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustrating an example of a schematicconfiguration of a continuous annealing line of a steel sheet accordingto a first embodiment of the present invention.

FIG. 2A is a longitudinal cross-sectional view illustrating an exampleof a configuration of an induction heating unit according to the firstembodiment of the present invention.

FIG. 2B is a longitudinal cross-sectional view illustrating an exampleof the configuration of the induction heating unit according to thefirst embodiment of the present invention.

FIG. 2C is a partial perspective view illustrating an example of theconfiguration of the induction heating unit according to the firstembodiment of the present invention.

FIG. 3 is a view illustrating an example of a configuration of an upperside heating coil and a lower side heating coil according to the firstembodiment of the present invention.

FIG. 4 is a view illustrating an example of a configuration of a controlunit of the induction heating unit according to the first embodiment ofthe present invention.

FIG. 5 is a view illustrating an example of a relationship between avoltage V_(c) at both ends of a capacitor of an MERS, a current I_(L)that flows to the induction heating unit, and an operation state of asemiconductor switch according to the first embodiment of the presentinvention.

FIG. 6A is a graph illustrating the relationship between frequency andtemperature ratio with respect to sheet conveyance speed, when power issupplied to the induction heating unit using the control unit accordingto the first embodiment of the present invention and a steel strip isheated.

FIG. 6B is a graph illustrating the relationship between frequency andtemperature ratio with respect to sheet conveyance speed, when power issupplied to the induction heating unit using a parallel resonance typeinverter in a conventional technique and the steel strip is heated.

FIG. 7 is a view illustrating an example of a configuration of a controlunit of an induction heating unit according to a second embodiment ofthe present invention.

FIG. 8A is a longitudinal cross-sectional view illustrating an exampleof a configuration of an induction heating unit according to a thirdembodiment of the present invention.

FIG. 8B is a longitudinal cross-sectional view illustrating an exampleof the configuration of the induction heating unit according to thethird embodiment of the present invention.

FIG. 8C is a partial perspective view illustrating an example of theconfiguration of the induction heating unit according to the thirdembodiment of the present invention.

FIG. 9A is a view illustrating an example of a configuration of ashielding plate according to the third embodiment of the presentinvention.

FIG. 9B is a schematic view illustrating an example of an eddy currentthat flows through a steel strip and the shielding plate according tothe third embodiment of the present invention.

FIG. 9C is a schematic view illustrating an example of a magnetic fieldthat is generated by the eddy current according to the third embodimentof the present invention.

FIG. 10A is a view illustrating an example of a temperature distributionof a conductive sheet, which is heated by the induction heating unit, inthe sheet width direction, in a case where the shielding plate accordingto the third embodiment of the present invention is used.

FIG. 10B is a view illustrating an example of a temperature distributionof a conductive sheet, which is heated by the induction heating unit, inthe sheet width direction, in a case where a shielding plate accordingto the first embodiment of the present invention is used.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the attached drawings. In each of the followingembodiments, a description will be made with respect to an example inwhich a transverse type induction heating unit and a control unitthereof are applied to a continuous annealing line of a steel sheet in amanufacturing line. In addition, in the following description,“transverse type induction heating unit” will be simply referred to as“induction heating unit” as necessary. In addition, unless particularlyspecified, in regard to attributes of the steel sheet (steel strip),values at room temperature (for example, 25° C.) will be used.

(First Embodiment)

First, a first embodiment of the present invention will be described.

<Schematic Configuration of Continuous Annealing Line>

FIG. 1 shows a side view illustrating an example of schematicconfiguration of a continuous annealing line of a steel sheet.

In FIG. 1, the continuous annealing line 1 includes a first container11, a second container 12, a third container 13, a first sealing rollerassembly 14, a conveyance unit 15, a second sealing roller assembly 16,a gas supply unit 17, rollers 19 a to 19 u, an induction heating unit20, and a control unit 100 of the induction heating unit. In addition,the induction heating unit 20 and the control unit 100 of the inductionheating unit make up an induction heating system.

The first sealing roller assembly 14 conveys (feeds) a steel strip 10into the first container 11 while shielding the first container 11 fromexternal air. The steel strip 10 conveyed into the first container 11 bythe first sealing roller assembly 14 is conveyed into the secondcontainer 12 by the rollers 19 a and 19 b in the first container 11. Thesteel strip 10 conveyed into the second container 12 is again conveyedinto the first container 11 by the rollers 19 g and 19 h while beingheated by the induction heating unit 20 which is disposed at both anupper side and a lower side of a horizontal portion of the secondcontainer 12 (of the steel strip 10 that is being conveyed). Here, theinduction heating unit 20 (heating coil thereof) is electricallyconnected to the control unit 100 of the induction heating units, and ACpower is supplied to the induction heating unit 20 from the control unit100 of the induction heating unit. An alternating magnetic field, whichintersects a sheet surface of the steel strip 10 in a substantiallyorthogonal manner, is generated by the AC power, and thereby the steelstrip 10 is inductively heated. In addition, details of a configurationof the induction heating unit 20 will be described later. In addition,in the following description, “electrical connection” will be simplyreferred to as “connection” as necessary.

The steel strip 10 that is returned into the first container 11 isconveyed to the conveyance unit 15 by the rollers 19 c to 19 f afterpassing through a soaking and slow cooling stage. The steel strip 10conveyed to the conveyance unit 15 is conveyed to the third container 13by the rollers 19 i and 19 j. The steel strip 10 conveyed to the thirdcontainer 13 is conveyed while being made to move in a vertically up anddown manner by the rollers 19 k to 19 u and is rapidly cooled in thethird container 13.

The second sealing roller assembly 16 forwards the steel strip 10, whichis rapidly cooled in this manner, to a subsequent process whileshielding the third container 13 from external air.

To the “first container 11, the second container 12, the third container13, and the conveyance unit 15” that make up a “conveying path of thesteel strip 10” described above, non-oxidation gas is supplied by thegas supply unit 17. In addition, the first container 11, the secondcontainer 12, the third container 13, and the conveyance unit 15 aremaintained in a non-oxidation gas atmosphere by the “first sealingroller assembly 14 and the second sealing roller assembly 16” thatshield the outside (external air) and the inside (the inside of thecontinuous annealing line 1).

<Configuration of Induction Heating Unit 20>

FIGS. 2A to 2C show views illustrating an example of a configuration ofan induction heating unit.

Specifically, FIG. 2A shows a view illustrating an example of theinduction heating unit 20 according to this embodiment, which is seenfrom a lateral direction of a line, and is a longitudinalcross-sectional view that is cut along the longitudinal direction (thevertical direction in FIG. 1) of the steel strip 10. In FIG. 2A, thesteel strip 10 is conveyed toward the left direction (refer to an arrowfacing from the right side to the left side in FIG. 2A). In addition,FIG. 2B shows a longitudinal cross-sectional view illustrating anexample of the induction heating unit 20 according to this embodiment,which is seen from an A-A′ direction in FIG. 1 (that is a view seen froma downstream in the sheet conveyance direction). In FIG. 2B, the steelstrip 10 is conveyed from the depth direction to the front direction. Inaddition, in FIGS. 2A and 2B, dimensions [mm] are also illustrated. Inaddition, FIG. 2C shows a partial perspective view illustrating a partof an example of the induction heating unit 20 according to thisembodiment. In FIG. 2C, a lower-right region shown in FIG. 2B (regionsurrounded by a broken line in FIG. 2B) is overlooked from an upper sideof the steel strip 10. However, in FIG. 2C, the second container 12 isomitted for easy understanding of the positional relationship between ashielding plate 31 and the steel strip 10.

In FIGS. 2A to 2C, the induction heating unit 20 includes an upper sideinductor 21 and a lower side inductor 22.

The upper side inductor 21 includes a core (magnetic core) 23, an upperside heating coil 24, and shielding plates 31 a and 31 c. The core 23may be configured by stacking a plurality of electrical steel sheets.

The upper side heating coil 24 is a conductor that is wound on the core23 through a slot (here, a depressed portion of the core 23) of the core23, and is a coil in which the number of turns is “1” (so-called singleturn). In addition, as shown in FIG. 2A, the upper side heating coil 24has a portion in which the shape of the longitudinal cross-sectionthereof is a hollow rectangle. A water-cooling pipe is connected to anend face of the hollow portion of the hollow rectangle. Cooling watersupplied from the water-cooling pipe flows to the hollow portion of thehollow rectangle (the inside of the upper side heating coil 24) andthereby the upper side inductor 21 is cooled. In addition, the shieldingplates 31 a and 31 c are attached on the bottom surface (slot side) ofthe core 23.

Similarly to the upper side inductor 21, the lower side inductor 22 isalso provided with a core (magnetic core) 27, a lower side heating coil28, and shielding plates 31 b and 31 d.

Similarly to the upper side heating coil 24, the lower side heating coil28 is a conductor that passes through a slot of the core 27 and is woundon the core 27, and is a coil in which the number of turns is “1”(so-called single turn). Furthermore, similarly to the upper sideheating coil 24, the lower side heating coil 28 has a portion in which ashape of a longitudinal cross-section thereof is a hollow rectangle. Awater-cooling pipe is connected to an end face of the hollow portion ofthe hollow rectangle, and cooling water can be made to flow to thehollow portion of the hollow rectangle. In addition, the shieldingplates 31 b and 31 d are installed on the upper surface (slot side) ofthe core 27.

In addition, a coil face (face on which a loop is formed and throughwhich a line of magnetic force penetrates) of the upper side heatingcoil 24 of the upper side inductor 21, and a coil face of the lower sideheating coil 28 of the lower side inductor 22 face each other with thesteel strip 10 interposed therebetween. Furthermore, sheet surfaces ofthe shielding plates 31 a to 31 d face end portions (edges) of the steelstrip 10 in the sheet width direction. To satisfy this positionalrelationship, the upper side inductor 21 is provided at an upper side(in the vicinity of the upper surface of a horizontal portion of thesecond container 12) compared to the steel strip 10, and the lower sideinductor 22 is provided at a lower side (in the vicinity of the lowersurface of the horizontal portion of the second container 12) comparedto the steel strip 10. In this embodiment, the shielding plates 31 a to31 d are copper plates that have a flat surface (refer to FIG. 2C). Theshielding plates 31 a to 31 d weaken the degree of electromagneticcoupling between the upper side heating coil 24 and the steel strip 10,and the degree of electromagnetic coupling between the lower sideheating coil 28 and the steel strip 10, thereby preventing the vicinityof the edges of the steel strip 10 in the steel width direction frombeing overheated.

In this manner, the upper side inductor 21 and the lower side inductor22 are different from each other in the position to be disposed, buthave the same configuration as each other. In addition, in thisconfiguration, since an alternating magnetic field generated from theheating coils intersects the conductive sheet 10 over the entire widththereof, the entire width of the conductive sheet 10 may be heated.

FIG. 3 shows a view illustrating an example of a configuration of theupper side heating coil 24 and the lower side heating coil 28. Inaddition, arrows shown in FIG. 3 illustrate an example of a direction inwhich a current flows.

As shown in FIG. 3, the upper side heating coil 24 includes copper pipes41 a and 41 b, and a copper bus bar (connection plate) 42 b that isconnected to base-end sides of the copper pipes 41 a and 41 b. Inaddition, the lower side heating coil 28 includes copper pipes 41 c and41 d, and a copper bus bar 42 f that is connected to base-end sides ofthe copper pipes 41 c and 41 d.

One output terminal of the control unit 100 of the induction heatingunit is connected to one end (front-end side of the copper pipe 41 a) ofthe upper side heating coil 24 through the copper bus bar 42 a. On theother hand, one end (front-end side of the copper pipe 41 c) of thelower side heating coil 28 is connected to the other end (front-end sideof the copper pipe 41 b) of the upper side heating coil 24 through thecopper bus bars 42 c to 42 e. In addition, the other output terminal ofthe control unit 100 of the induction heating unit is connected to theother end (front-end side of the copper pipe 41 d) of the lower sideheating coil 28 through copper bus bars 42 i, 42 h, and 42 g.

As described above, the upper side heating coil 24 and the lower sideheating coil 28 are connected in series to the control unit 100 of theinduction heating unit by combining the copper pipes 41 a to 41 d andthe copper bus bars 42 a to 42 i, thereby forming coils in which thenumber of turns is “1”. Here, the direction (in FIG. 3, a clockwiserotation) of a loop of a current that flows through the upper sideheating coil 24 is the same as the direction of a loop of a current thatflows through the lower side heating coil 28.

In addition, as described later, the control unit 100 of the inductionheating unit supplies AC power to the upper side heating coil 24 and thelower side heating coil 28 of the induction heating unit 20. Therefore,in FIG. 3, the control unit 100 of the induction heating unit isindicated as an AC power supply.

In addition, here, for illustrating a configuration of the upper sideheating coil 24 and the lower side heating coil 28 in an easy manner,the copper pipes 41 a to 41 d and the copper bus bars 42 a to 42 i areconnected in a manner as shown in FIG. 3. However, to wind the upperside heating coil 24 and the lower side heating coil 28 on the cores 23and 27, respectively, it is necessary for the copper pipes 41 a to 41 dto pass through (to be attached to) the slots of the cores 23 and 27.Therefore, actually, the copper bus bars 42 a to 42 g are installed tothe copper pipes 41 a to 41 d at portions other than portions in whichthe copper pipes 41 a to 41 d are installed to the cores 23 and 27.

<Configuration of Control Unit 100 of Induction Heating Unit>

FIG. 4 shows a view illustrating an example of a configuration of thecontrol unit 100 of the induction heating unit. In addition, in thefollowing description, “control unit of the induction heating unit” issimply referred to as “control unit” as necessary.

In FIG. 4, the control unit 100 includes an AC power supply 160, arectifying unit 110, a reactor 120, a magnetic energy recoverybidirectional current switch (MERS; Magnetic Energy Recovery Switch)130, a gate control unit 140, an output current setting unit 150, acurrent transformer 170, and a frequency setting unit 180. Here, thecurrent transformer 170 is used as a current measuring unit thatmeasures the value of an alternating current that flows to the inductionheating unit. In addition, in the following description, “magneticenergy recovery switch” is referred to as “MERS” as necessary.

In FIG. 4, the AC power supply 160 is connected to an input terminal ofthe rectifying circuit 110. One end of the reactor 120 is connected toone end of the rectifying circuit 110 on an output side, and a DCterminal c of the MERS 130 is connected to the other end of therectifying circuit 110. The other end of the reactor 120 is connected toa DC terminal b of the MERS 130. The rectifying circuit 110 rectifies ACpower supplied from the AC power supply 160 and applies DC power to theMERS 130 through the reactor 120. The rectifying circuit 110 isconfigured by using, for example, a thyristor. As described above, inthis embodiment, for example, a power supply unit is realized using theAC power supply 160 and the rectifying circuit 110. This power supplyunit is a unit that supplies DC power described later to the DCterminals b and c of the MERS 130 in FIG. 4. Therefore, a DC powersupply such as a battery that has a current control function may be usedas the power supply unit.

[Configuration of MERS 130]

Hereinafter, an example of a configuration of the MERS 130 will bedescribed.

The MERS 130 converts DC power, which is input from the rectifyingcircuit 110 through the reactor 120, to AC power according to a methoddescribed later, and outputs the AC power to the induction heating unit20.

In FIG. 4, the MERS 130 includes a bridge circuit that is configuredusing first to fourth reverse conductivity type semiconductor switches131 to 134, and a capacitor C having a polarity. This capacitor C isconnected between the DC terminals b and c of the bridge circuit, and apositive electrode (+) of the capacitor C is connected to the DCterminal b.

The other end of the reactor 120 is connected to the DC terminal b, andthe other end of the rectifying circuit 110 on the output side isconnected to the DC terminal c. In addition, one end (copper bus bar 42a) and the other end (copper bus bar 42 g) of the induction heating unit20 are connected to the AC terminals a and d (refer to FIG. 3),respectively.

The bridge circuit of the MERS 130 includes a first path L1 reaching theAC terminal d from the AC terminal a through the DC terminal b, and asecond path L2 reaching the AC terminal d from the AC terminal a throughthe DC terminal c. The first reverse conductivity type semiconductorswitch 131 is connected between the AC terminal d and the DC terminal b,and the fourth reverse conductivity type semiconductor switch 134 isconnected between the DC terminal b and the AC terminal a. In addition,the second reverse conductivity type semiconductor switch 132 isconnected between the AC terminal d and the DC terminal c, and the thirdreverse conductivity type semiconductor switch 133 is connected betweenthe DC terminal c and the AC terminal a. In this manner, the first andsecond reverse conductivity type semiconductor switches 131 and 132 areconnected in parallel, and the third and fourth reverse conductivitytype semiconductor switches 133 and 134 are connected in parallel. Inaddition, the first and fourth reverse conductivity type semiconductorswitches 131 and 134 are connected in series, and the second and thirdreverse conductivity type semiconductor switches 132 and 133 areconnected in series.

Each of the first to fourth reverse conductivity type semiconductorswitches 131 to 134 allows a current to flow in one direction at thetime of a switch-off in which an on-signal is not input to a gateterminal thereof, and allows a current to flow in both directions at thetime of a switch-on in which the on-signal is input to the gateterminal. That is, the reverse conductivity type semiconductor switches131 to 134 allows a current to flow only in one direction between asource terminal and a drain terminal at the time of the switch-off butallows a current to flow in both directions between the source terminaland the drain terminal at the time of the switch-on. In addition, in thefollowing description, “a direction toward which each of the reverseconductivity type semiconductor switches 131 to 134 allows a current toflow at the time of the switch-off” is also referred to as “a switchforward direction” as necessary. In addition, “a direction toward whicheach of the reverse conductivity type semiconductor switches 131 to 134does not allow a current to flow at the time of the switch-off” is alsoreferred to as “a switch reverse direction” as necessary. Furthermore,in the following description, “a connection direction with respect tothe bridge circuit in the switch forward direction and the switchreverse direction” is also referred to as “a switch polarity” asnecessary.

In addition, each of the reverse conductivity type semiconductorswitches 131 to 134 is disposed to satisfy the switch polarity asdescribed below. The first reverse conductivity type semiconductorswitch 131 and the second reverse conductivity type semiconductor switch132, which are connected in parallel, have switch polarities opposite toeach other. Similarly, the third reverse conductivity type semiconductorswitch 133 and the fourth reverse conductivity type semiconductor switch134, which are connected in parallel, have switch polarities opposite toeach other. In addition, the first reverse conductivity typesemiconductor switch 131 and the fourth reverse conductivity typesemiconductor switch 134, which are connected in series, have switchpolarities opposite to each other. Similarly, the second reverseconductivity type semiconductor switch 132 and the third reverseconductivity type semiconductor switch 133, which are connected inseries, have switch polarities opposite to each other. Therefore, thefirst reverse conductivity type semiconductor switch 131 and the thirdreverse conductivity type semiconductor switch 133 have the same switchpolarity as each other. Similarly, the second reverse conductivity typesemiconductor switch 132 and the fourth reverse conductivity typesemiconductor switch 134 have the same switch polarity as each other. Inaddition, the switch polarity of the first and third reverseconductivity type semiconductor switches 131 and 133 is opposite to thatof the second and fourth reverse conductivity type semiconductorswitches 132 and 134.

In addition, in regard to the switch polarities shown in FIG. 4, theswitch polarity of the first and third reverse conductivity typesemiconductor switches 131 and 133, and the switch polarity of thesecond and fourth reverse conductivity type semiconductor switches 132and 134 may be reversed to each other.

In addition, various configurations may be considered with respect tothe first to fourth reverse conductivity type semiconductor switches 131to 134, but in this embodiment, the first to fourth reverse conductivitytype semiconductor switches 131 to 134 are configured by a parallelconnection between semiconductor switches S1 to S4 and diodes D1 to D4,respectively. That is, each of the first to fourth reverse conductivitytype semiconductor switches 131 to 134 includes one diode (correspondingone among diodes D1 to D4) and one semiconductor switch (correspondingone among semiconductor switches S1 to S4) that is connected to thediode in parallel.

In addition, respective gate terminals G1 to G4 of the semiconductorswitches S1 to S4 are connected to the gate control unit 140. Anon-signal, which allows the semiconductor switches S1 to S4 to be turnedon, is input to the gate terminals G1 to G4 from the gate control unit140 as a control signal to the MERS 130. In a case where the on-signalis input, the semiconductor switches S1 to S4 enter an on-state, and mayallow a current to flow in a both direction. However, in a case wherethe on-signal is not input, the semiconductor switches S1 to S4 enter anoff-state, and can not allow a current to flow in any direction.Therefore, when the semiconductor switches S1 to S4 are turned off, acurrent can flow only in the conduction direction (forward direction) ofthe diodes D1 to D4 that are connected in parallel to the semiconductorswitches S1 to S4.

In addition, the reverse conductivity type semiconductor switchesincluded in the MERS 130 are not limited to the first to fourth reverseconductivity type semiconductor switches 131 to 134. That is, anyreverse conductivity type semiconductor switch is preferable as long asthis switch has a configuration capable of showing the above-describedoperation. For example, the reverse conductivity type semiconductorswitches may have a configuration using a switching element such as apower MOSFET and a reverse conducting GTO thyristor, or may have aconfiguration in which a semiconductor switch such as an IGBT and adiode are connected in parallel.

In addition, hereinafter, a description will be made by substituting theswitch polarity of the first to fourth reverse conductivity typesemiconductor switches 131 to 134 with the polarity of the diodes D1 toD4. A switch forward direction (direction toward which a current flowsat the time of a switch-off) is a conduction direction (forwarddirection) of each of the diodes D1 to D4, and a switch reversedirection (direction toward which a current does not flow at the time ofthe switch-off) is a non-conduction direction (reverse direction) ofeach of the diodes D1 to D4. In addition, conduction directions betweendiodes (D1 and D2, or D3 and D4) connected in parallel are opposite toeach other, and conduction direction between diode (D1 and D4, or D2 andD3) connected in series are opposite to each other. In addition,conduction directions of the diodes D1 and D3 are the same as eachother. Similarly, conduction directions of the diodes D2 and D4 are thesame as each other. Therefore, the conduction direction of the diode D1and D3 and the conduction direction of the diodes D2 and D4 are oppositeto each other. In addition, the conduction directions of thesemiconductor switches S1 to S4 and the diodes D1 to D4 are set with adirection of a

[Operation of MERS 130]

FIG. 5 shows a view illustrating an example of a relationship between avoltage V_(c) at both ends of a capacitor C of the MERS 130, a currentI_(L) that flows to the induction heating unit 20, and an operationstate of the semiconductor switches S1 to S4.

In FIG. 5, for a period in which a waveform rises on a side indicated as“S1·S3 gate”, the switches S1 and S3 are in an on-state, and thesemiconductor switches S2 and S4 are in an off-state. In addition, for aperiod in which a waveform rises on a side indicated as “S2·S4 gate”,the semiconductor switches S2 and S4 are in an on-state, and theswitches S1 and S3 are in an off-state. For a period in which a waveformdoes not rise on either the “S1·S3 gate” side or the “S2·S4 gate” side,all of the semiconductor switches S1 to S4 are in an off-state. In thismanner, when the semiconductor switch S1 is turned on (off), thesemiconductor switch S3 is turned on (off), and therefore thesemiconductor switches S1 and S3 operate in conjunction with each other.Similarly, when the semiconductor switch S2 is turned on (off), thesemiconductor switch S4 is turned on (off), and therefore thesemiconductor switches S2 and S4 operate in conjunction with each other.Hereinafter, an example of the operation of the MFRS 130 will bedescribed with reference to FIGS. 4 and 5.

As shown in FIG. 5, an initial stage of a period A is a dead timeaccompanying a switch operation, and for this dead time, not only thesemiconductor switches S1 and S3 but also the semiconductor switches S2and S4 are turned off For this dead time, a current flows through thepath of the diode D4→the capacitor C→the diode D2, and thereforecharging of the capacitor C is initiated. As a result, the voltage V_(c)at both ends of the capacitor C is raised, and therefore the currentI_(L) (absolute value thereof) flowing to the induction heating unit 20decreases. When the semiconductor switches S2 and S4 are turned on(while the semiconductor switches S1 and S3 are turned off) before thecharging of the capacitor C is completed, a current flows through a pathof the semiconductor switch S4 and the diode D4→the capacitor C→thesemiconductor switch S2 and the diode D2, and therefore the capacitor Cis charged (period A). That is, in this period A, the voltage V_(c) atboth ends of the capacitor C is raised, and therefore the current I_(L)(absolute value thereof) flowing to the induction heating unit 20decreases.

When the charging of the capacitor C is completed, the current I_(L)flowing to the induction heating unit 20 becomes zero. When thesemiconductor switches S2 and S4 are turned on until the charging of thecapacitor C is completed, and then the charging of the capacitor C iscompleted, the energy (charge) charged in the capacitor C is output(discharged) through the semiconductor switches S4 and S2. As a result,the current I_(L) flows through a path of the semiconductor switchS4→the induction heating unit 20→the semiconductor switch S2 (period B).That is, in this period B, the voltage V_(c) at both ends of thecapacitor C is lowered, and therefore the current I_(L) (absolute valuethereof) flowing to the induction heating unit 20 increases.

When the discharging of the capacitor C is completed, the voltage V_(c)at both ends of the capacitor C becomes zero, and therefore a reversevoltage is not applied to the diodes D1 and D3. Therefore, the diodes D1and D3 enter a conduction state, and the current I_(L) flows through apath of the semiconductor switch S4→the induction heating unit 20→thediode D1 and a path of the diode D3→the induction heating unit 20→thesemiconductor switch S2 in parallel (period C). The current I_(L)circulates between the induction heating unit 20 and the MERS 130.Therefore, in the period C, the absolute value of the current I_(L) isattenuated in response to a time constant that is determined byimpedance of the upper side heating coil 24, the lower side heating coil28, and the steel strip 10 that is an object to be heated.

Then, in the dead time, not only the semiconductor switches S1 and S3,but also the semiconductor switches S2 and S4 are turned off. For thedead time, a current flows through a path of the diode D1→the capacitorC→the diode D3, and therefore the charging of the capacitor C isinitiated (period D). As a result, the voltage V_(c) at both ends of thecapacitor C is raised, and therefore the current I_(L) (absolute valuethereof) flowing to the induction heating unit 20 decreases. When thesemiconductor switches S1 and S3 are turned on (while the semiconductorswitches S2 and S4 are turned off) before the charging of the capacitorC is completed, a current flows through the path of the semiconductorswitch S1 and the diode D1→the capacitor C→the semiconductor switch S3and the diode D3, and therefore the capacitor C is charged (period D).That is, in this period D, the voltage V_(c) at both ends of thecapacitor C is raised, and therefore the current I_(L) (absolute valuethereof) flowing to the induction heating unit 20 decreases.

When the charging of the capacitor C is completed, the current I_(L)flowing to the induction heating unit 20 becomes zero. When thesemiconductor switches S1 and S3 are turned on until the charging of thecapacitor C is completed, and then the charging of the capacitor C iscompleted, the energy (charge) charged in the capacitor C is output(discharged) through the semiconductor switches S1 and S3. As a result,the current I_(L) flows through a path of the semiconductor switchS1→the induction heating unit 20→the semiconductor switch S3 (period E).That is, in this period E, the voltage V_(c) at both ends of thecapacitor C is lowered, and therefore the current I_(L) (absolute valuethereof) flowing to the induction heating unit 20 increases.

When the discharging of the capacitor C is completed, the voltage V_(c)at both ends of the capacitor C becomes zero, and therefore a reversevoltage is not applied to the diodes D2 and D4. Therefore, the diodes D2and D4 enter a conduction state, and the current I_(L) flows through apath of the semiconductor switch S1→the induction heating unit 20→thediode D4 and a path of the diode D2→the induction heating unit 20→thesemiconductor switch S3 in parallel (period F). The current I_(L)circulates between the induction heating unit 20 and the MERS 130.Therefore, in the period F, the absolute value of the current I_(L) isattenuated in response to a time constant that is determined byimpedance of the upper side heating coil 24, the lower side heating coil28, and the steel strip 10 that is an object to be heated. Then, itreturns to the operation for the period A, and the operations for theperiods A to F are repetitively performed.

As described above, when turn-on and turn-off (switching operation)timings (times) of the respective gate terminals G1 to G4 (G1 and G3,and G2 and G4) of the semiconductor switches S1 to S4 (S1 and S3, and S2and S4) are adjusted, a current of a desired frequency can be made toflow through the induction heating unit 20 (the upper side heating coil24 and the lower side heating coil 28), thereby realizing frequencycontrol type induction heating. That is, due to the gate control unit140 that adjusts the conduction timing of the semiconductor switches S1to S4, a frequency of the current I_(L) that flows to the inductionheating unit 20 that is a load can be controlled to an arbitrary value.In addition, when capacitance C_(p) of the capacitor C is determinedaccording to Equation (1) described below, the period in which thevoltage V_(c) at both ends of the capacitor C is zero can be adjusted.C _(p)=1/[(2×π×f _(t))² ×L]  (1)

Here, C_(p) represents capacitance (F) of the capacitor C, and Lrepresents inductance (H) of loads including the induction heating unit20. In addition, f_(t) represents an apparent frequency (Hz) withrespect to the capacitor C, which is expressed by Equation (2) describedbelow.f _(t)=1/(2×t+1/f)  (2)

Here, t represents a period (sec) in which the voltage V_(c) at bothends of the capacitor C is zero, and f represents a frequency (Hz) ofthe voltage V_(c) and the current I_(L) in a case where a period inwhich the voltage V_(c) at both ends of the capacitor C is zero is notpresent. When a capacitor C, which has capacitance C_(p) that isobtained by substituting f_(t) (that is, f) when t is zero in Equation(2) into Equation (1), is selected, a period in which the voltage V_(c)at both ends of the capacitor C is zero is not present.

[Configuration of Frequency Setting Unit 180]

Returning to the description of FIG. 4, an example of a configuration ofthe frequency setting unit 180 will be described. The frequency settingunit 180 is a unit that sets the frequency (output frequency) of ACpower to be supplied to the induction heating unit 20. To realize thefunction thereof, the frequency setting unit 180 includes anobject-to-be-heated information acquiring unit 181, a frequency settingtable 182, and a frequency selector 183.

The object-to-be-heated information acquiring unit 181 acquiresattribute information of the steel strip 10 that is an object to beheated. For example, the object-to-be-heated information acquiring unit181 acquires (receives) the attribute information from an externalcomputer that is an input unit through a network, or acquires (input)the attribute information on the basis of information that is input by auser with respect to a user interface (one of input units) provided forthe control unit 100. Here, the attribute information of the steel strip10 is information that is capable of specifying a relative permeability,a resistance, and a sheet thickness of the steel strip 10. For example,the relative permeability, the resistance, and the sheet thicknessitself of the steel strip 10 may be set as the attribute information, orin a case where the relative permeability, the resistance, and the sheetthickness itself of the steel strip 10 are determined according tospecifications, a name (a trade name or the like) of the steel strip 10having the specifications may be set as the attribute information.

The frequency selector 183 uses the attribute information acquired bythe object-to-be-heated information acquiring unit 181 as a key andselects one frequency among frequencies registered in the frequencysetting table 182. In the frequency setting table 182, the attributeinformation and the frequency are correlated with each other and areregistered in advance.

Information of a frequency (output frequency) selected by the frequencyselector 183 is transmitted to the gate control unit 140. The gatecontrol unit 140 determines turn-on and turn-off (switching operation)timings of the respective gate terminals G1 to G4 of the semiconductorswitches S1 to S4 of the MERS 130 so that AC power of the selectedfrequency is generated, and outputs an on-signal to a gate terminal of asemiconductor switch to be turned on. In this manner, the MERS 130outputs the AC power of the frequency (the output frequency) that is setto the gate control unit 140 by the frequency setting unit 180 to theinduction heating unit 20 as described above.

As described above, in this embodiment, the frequency (the outputfrequency) of the AC power to be supplied to the induction heating unit20 is automatically determined in response to the relative permeability,the resistance, and the sheet thickness of the steel strip 10. This isbased on a finding obtained through various experiments performed by theinventors, specifically, a finding that the temperature distribution(particularly, the temperature in the vicinity of an edge) of the steelstrip 10 is affected by the frequency of the AC power supplied to theinduction heating unit 20, the attribute information (the relativepermeability, the resistance, and the sheet thickness) of the steelstrip 10 that is an object to be heated, and a gap (distance between theupper side heating coil 24 and the lower side heating coil 28).

Hereinafter, the reason why this phenomenon occurs will be described.

First, a description will be made with respect to a case where thetemperature of the steel strip 10 is equal to or higher than the Curietemperature.

When the steel strip 10 is at a temperature that is equal to or higherthan the Curie temperature, a main magnetic field that is generated fromthe induction heating unit 20 penetrates through the steel strip 10, andan eddy current within the steel strip 10 (within a plane orthogonal tothe sheet thickness) increases. This eddy current is repelled from amain magnetic field and is apt to be biased to the vicinity of the edgeof the steel strip 10. Therefore, a high-temperature region is apt tooccur in the vicinity of the edge of the steel strip 10.

Here, the eddy current within the steel strip 10 is proportional to across-sectional area (cross-sectional area including a sheet thicknessdirection) of the steel strip 10, such that in a case where the sheetthickness of the steel strip 10 is large, the cross-sectional area ofthe steel strip 10 becomes large and therefore the eddy current withinthe steel strip 10 increases.

In addition, the eddy current of the steel strip 10 is inverselyproportional to a resistance of the steel strip 10, such that in a casewhere the resistance of the steel strip 10 is small, the eddy currentwithin the steel strip 10 increases.

In addition, a frequency of AC power supplied to the induction heatingunit 20 is proportional to an induced electromotive force that isgenerated within the steel strip 10 due to the main magnetic fieldgenerated from the induction heating unit 20. The eddy current of thesteel strip 10 is proportional to the induced electromotive force, suchthat in a case where the frequency of the AC power supplied to theinduction heating unit 20 is high, the eddy current within the steelstrip 10 increases.

In addition, in a case where the gap is small, the main magnetic fieldgenerated from the induction heating unit 20 becomes large, such thatthe induced electromotive force generated within the steel strip 10 dueto the main magnetic field becomes large and therefore the eddy currentwithin the steel strip 10 increases.

Next, a description will be made with respect to a case where thetemperature of the steel strip 10 is less than Curie temperature.

In a case where the temperature of the steel strip 10 is less than Curietemperature, a relative permeability of the steel strip 10 is large,such that the main magnetic field generated from the induction heatingunit 20 is difficult to penetrate through the steel strip 10 andtherefore bypasses the edge portion of the steel strip 10. As a result,in the vicinity of the edge of the steel strip 10 in the sheet widthdirection, the current density of the eddy current becomes large, andtherefore a high temperature region occurs in the vicinity of the edgeof the steel strip 10 in the sheet width direction.

As described above, factors (the frequency of the AC power supplied tothe induction heating unit 20, the relative permeability, resistance,and sheet thickness of the steel strip 10 that is an object to beheated, and the gap), which have an effect on the temperature of thesteel strip 10, are independent from each other. Among these factors,the relative permeability, resistance, and sheet thickness of the steelstrip 10, and the gap are determined by operational conditions (hardwarerestrictions on a material that is an object to be heated and afacility). Therefore, in this embodiment, among these factors, “thefrequency (the output frequency) of the AC power supplied to theinduction heating unit 20” that can be controlled through on-line ischanged using the frequency setting unit 180 to adjust the temperatureof the steel strip 10.

In addition, as is the case with this embodiment, when all of therelative permeability, the resistance, and the sheet thickness of thesteel strip 10, and the frequency are correlated with each other and areregistered in the frequency setting table 182, the temperaturedistribution of the steel strip 10 in the sheet width direction can beadjusted in a relatively uniform manner. Therefore, it is preferablethat all of the relative permeability, resistance, and sheet thicknessof the steel strip 10, and the frequency be correlated with each other.However, it is not necessary to correlate all of the relativepermeability, resistance, and sheet thickness of the steel strip 10, andthe frequency, and at least one of the relative permeability,resistance, and sheet thickness of the steel strip 10 may be correlatedwith the frequency in the frequency setting unit 180. In addition, atleast one of the relative permeability, resistance, and sheet thicknessof the steel strip 10, and the gap may be correlated with the frequency.

[Configuration of Output Current Setting Unit 150]

The output current setting unit 150 is a unit that sets a magnitude(output current value) of the AC current I_(L) supplied to the inductionheating unit 20. To realize this function, the output current settingunit 150 includes an object-to-be-heated information acquiring unit 151,an output current setting table 152, and an output current selector 153.

The object-to-be-heated information acquiring unit 151 acquiresattribute information of the steel strip 10 that is an object to beheated, similarly to the object-to-be-heated information acquiring unit181.

The output current selector 153 uses the attribute information acquiredby the object-to-be-heated information acquiring unit 151 as a key andselects one current value among current values registered in the outputcurrent setting table 152. In the output current setting table 152, theattribute information and the current value are correlated with eachother and are registered in advance. In addition, a control angle of therectifying unit 110 is set in response to a difference between thecurrent value (the output current value) selected by the output currentselector 153 and a current value measured by the current transformer170. In the case of adopting a thyristor rectifying device as therectifying unit 110, a gate firing angle of the thyristor is set. Inthis manner, the value of the current flowing to the induction heatingunit 20 is fed back and the control angle (the gate firing angle) of therectifying unit 110 is controlled, such that the value of the currentflowing to the induction heating unit 20 may be constantly controlled tobe the current value (output current value) selected by the outputcurrent selector 153. As a result, the power supply unit (the AC powersupply 160 and the rectifying unit 110) supplies DC power to the MFRS130, and therefore the alternating current measured by the currenttransformer 170 can be adjusted to the current value (the output currentvalue) set by the output current setting unit.

As described above, in this embodiment, the current value (the outputcurrent value) of the AC power supplied to the induction heating unit 20is automatically determined in response to the relative permeability,resistance, and sheet thickness of the steel strip 10. This is becausethe current value corresponding to a target temperature can bedetermined by the relative permeability, the resistance, and the sheetthickness of the steel strip 10.

In addition, similarly to this embodiment, when all of the relativepermeability, resistance, and sheet thickness of the steel strip 10, andthe current value are correlated with each other and are registered inthe output current setting table 152, a temperature distribution and anaverage temperature of the steel strip 10 in the sheet width directionmay be set in a relatively appropriate manner. Therefore, it ispreferable that all of the relative permeability, the resistance, andthe sheet thickness of the steel strip 10, and the current value becorrelated with each other. However, it is not necessary to correlateall of the relative permeability, resistance, and sheet thickness of thesteel strip 10 with the current value, and at least one of the relativepermeability, resistance, and sheet thickness of the steel strip 10 andthe current value may be correlated with each other in the outputcurrent setting unit 150. In addition, at least one of the relativepermeability, resistance, and sheet thickness of the steel strip 10, andthe gap may be correlated with the current value.

<Effect of This Embodiment>

FIG. 6A shows a graph illustrating the relationship between frequencyand temperature ratio with respect to sheet conveyance speed, when poweris supplied to the induction heating unit 20 using the control unit 100according to the embodiment and a steel strip 10 is heated. In addition,FIG. 6B shows a graph illustrating the relationship between frequencyand temperature ratio with respect to a sheet conveyance speed, whenpower is supplied to the induction heating unit 20 using a parallelresonance type inverter in a conventional technique and the steel strip10 is heated. Here, a temperature ratio (temperature ratio ofedge/center) is a value obtained by dividing a temperature in an endportion (edge) of the steel strip 10 in the sheet width directionthereof by a temperature in a central portion of the steel strip 10 inthe sheet width direction thereof. The more the value of the temperatureratio approaches 1, the more uniform the temperature distribution of thesteel strip 10 in the sheet width direction is. In addition, thefrequency is a frequency of a current applied to the induction heatingunit 20. In addition, specifications of the steel strip 10 are asfollows.

<Specifications of Steel Strip>

•Material: Stainless steel sheet •Sheet Thickness: 0.3 mm •Width: 500 mm

As shown in FIG. 6A, when the control unit 100 according to thisembodiment is used, even in a case where the sheet conveyance speedvaries, the frequency of the current, which may be applied to theinduction heating unit 20, may be held substantially constant, andtherefore the temperature ratio can be controlled to be substantiallyconstant.

On the other hand, when the sheet conveyance speed varies, the impedanceof the load varies, such that in a case where the parallel resonancetype inverter in the conventional technique is used, the inverter of thevoltage source controls the output frequency of the inverter in such amanner that a resonance condition of the load is maintained. Therefore,as shown in FIG. 6B, the output frequency of the inverter varies inresponse to a variation of the impedance of the load. As a resultthereof, the temperature ratio varies significantly and therefore thetemperature ratio can not be controlled to be constant.

As described above, according to this embodiment, the current I_(L) ofthe frequency (the output frequency) corresponding to the attribute(attribute information) of the steel strip 10 is supplied to theinduction heating unit 20 using the MERS 130. Therefore, the controlunit according to this embodiment is not subjected to a restriction inregard to an operation with a resonant frequency like the conventionaltechnique, such that even when the sheet conveyance speed of the steelstrip 10 varies, the frequency of the current I_(L) that is supplied tothe induction heating unit 20 may be set to a desired value in responseto the attribute of the steel strip 10. Therefore, when the conductivesheet is heated using the transverse type induction heating unit, evenwhen the sheet conveyance speed of the conductive sheet varies, it ispossible to prevent the temperature distribution of the conductive sheetin the sheet width direction from being nonuniform. In addition, thecurrent I_(L) of a frequency, which is appropriate to the steel strip 10that is an object to be heated (particularly, which makes thetemperature distribution in the sheet width direction as uniform aspossible), may be set to the induction heating unit 20.

In addition, in this embodiment, the control angle of the rectifyingunit 110 is changed in response to the attribute of the steel strip 10,and therefore the current I_(L) having a magnitude corresponding to theattribute of the steel strip 10 is supplied to the induction heatingunit 20. As a result, the current I_(L) having a magnitude appropriateto the steel strip 10 that is an object to be heated can flow throughthe induction heating unit 20. In addition, since the frequency iscontrolled to be constant, the temperature distribution of theconductive sheet in the sheet width direction can be uniformlycontrolled without actually measuring the variation in temperature withthe passage of time at various positions of the steel strip 10.

Furthermore, in regard to the induction heating system provided with thecontrol unit 100 and the induction heating unit 20 having the shieldingplates 31 a to 31 d, since even when the sheet conveyance speed varies,the frequency of the AC power does not vary, it is not necessary toconsider a variation (variation with the passage of time) in the eddycurrent generated at the edge portion of the steel strip 10. Therefore,when the control unit 100 is used in the induction heating system, evenwhen the operational conditions vary, a heating amount in the vicinityof the edge can be appropriately controlled by the shielding plates 31 ato 31 d.

(Second Embodiment)

Next, a second embodiment of the present invention will be described. Inthe above-described first embodiment, the alternating current I_(L) ismade to flow to the induction heating unit 20 directly from the MERS130. Conversely, according to this embodiment, the alternating currentI_(L) is made to flow to the induction heating unit 20 from the MERS 130through a transformer. In this manner, in a configuration of thisembodiment, the transformer is added to the above-describedconfiguration of the first embodiment. Therefore, in this embodiment,the same reference symbols as those given in FIG. 1 to FIG. 6B will begiven to the same portions as the above-described first embodiment, anda detailed description thereof will be omitted here.

FIG. 7 shows a view illustrating an example of a configuration of acontrol unit 200 of an induction heating unit.

As shown in FIG. 7, the control unit 200 according to this embodimentfurther includes an output transformer 210 compared to the control unit100 according to the first embodiment shown in FIG. 4.

A primary side (input side) terminal of the output transformer 210 isconnected to the AC terminals a and d of the MERS 130. A secondary side(output side) terminal of the output transformer 210 is connected to theinduction heating unit 20 (the copper bus bars 42 a and 42 g). Thetransformation ratio (input:output) of the output transformer 210 is N:1(N>1).

As described above, in this embodiment, since the output transformer 210having the transformation ratio of N:1 (N>1) is disposed between theMERS 130 and the induction heating unit 20, substantially N timescurrent of the current flowing through the MERS 130 can be made to flowto the induction heating unit 20. Therefore, in this embodiment, a largecurrent can be made to flow to the induction heating unit 20 withoutmaking a large current flow to the “semiconductor switches S1 to S4 andthe diodes D1 to D4” that make up the MERS 130.

In addition, a plurality of taps may be provided on the primary side orthe secondary side of the output transformer 210 in such a manner thatthe transformation ratio of the output transformer 210 can be changed,and the tap to be used may be properly used in response to the steelstrip 10 that is an object to be heated.

(Third Embodiment)

Next, a third embodiment of the present invention will be described. Inthe above-described first and second embodiments, a flat plate is usedas the shielding plates 31 a to 31 d provided for the induction heatingunit 20. Conversely, in this embodiment, a depressed portion is formedin the shielding plates provided for the induction heating unit 20. Inthis manner, this embodiment and the above-described first and secondembodiments are different in a part of a configuration of the shieldingplates. Therefore, in this embodiment, the same reference symbols asthose given in FIG. 1 to FIG. 7 will be given to the same portions asthe above-described first and second embodiments, and a detaileddescription thereof will be omitted here.

FIGS. 8A to 8C show views illustrating an example of a configuration ofthe induction heating unit. FIG. 8A, FIG. 8B, and FIG. 8C correspond toFIG. 2A, FIG. 2B, and FIG. 2C, respectively. Instead of the shieldingplates 31 a to 31 d shown in FIGS. 2A to 2C, shielding plates 301 a to301 d are used. In addition, the shielding plates 301 a to 301 d aredisposed at positions shown in FIG. 8B in such a manner that thedepressed portion described later faces (is opposite to) the steel strip10 (in the second container 12). In addition, the induction heating unitincludes an upper side inductor 201 and a lower side inductor 202. Inaddition, the upper side inductor 201 and the lower side inductor 202are substantially the same as the upper side inductor 21 and the lowerside inductor 22 shown in FIGS. 2A to 2C, respectively, except for theconfiguration of the shielding plates.

In addition, FIGS. 9A to 9C show views illustrating an example of aconfiguration of the shielding plate 301 (shielding plates 301 a to 301d). Specifically, FIG. 9A shows a perspective view taken by overlookingthe shielding plate 301 from an upper side. In addition, FIG. 9B shows aview taken by overlooking a region of the shielding plate 301 d shown inFIG. 8C from immediately above the steel strip 10. In addition, FIG. 9Bshows only a portion that is necessary to explain a positionalrelationship between the steel strip 10 and the shielding plate 301 d.In addition, FIG. 9C shows a schematic view illustrating an example of amagnetic field that is generated between the shielding plates 301 a, 301b and the steel strip 10. However, in FIGS. 9B and 9C, the secondcontainer 12 is omitted for easy understanding of an effect of theshielding plates 301 a to 301 d.

As shown in FIG. 9A, the shielding plate 301 includes a main shieldingplate 50 a and a rear plate 50 b.

The width and length of the main shielding plate 50 a are the same asthose of the rear plate 50 b. However, the rear plate 50 b is formed ofa copper plate in which a longitudinal cross-section and a transversecross-section are uniform, and conversely, the main shielding plate 50 ais formed of a copper plate in which two rhombic holes are formed in thelongitudinal direction thereof. The shielding plate 301 is formed byclose contact between the main shielding plate 50 a and the rear plate50 b, and has two rhombic depressed portions (non-penetration holes) 51and 52 in the longitudinal direction. In addition, in FIG. 9A,dimensions [mm] related to the positions at which the depressed portions51 and 52 are disposed are also indicated.

As shown in FIGS. 9B and 9C, the shielding plate 301 is installed on thebottom surface (slot side) of the core 23 and the top surface (slotside) of the core 27 in such a manner that a surface in which thedepressed portions 51 and 52 are formed faces the steel strip 10.

In this embodiment, as shown in FIG. 9B, the depressed portions 51 and52 of the shielding plate 301 (301 d) and a sheet surface of the steelstrip 10 are opposite to each other in the vicinity of an edge 10 a ofthe steel strip 10 in the sheet width direction. Specifically, a regionthat is located on the edge 10 a side compared to the maximum currentpassing region 56 faces the depressed portions 51 and 52 of theshielding plate 301. The region that is located on the edge 10 a sideincludes a region between a maximum current passing region 56 that is aregion in which an eddy current flowing through the steel strip 10becomes maximum by operating the induction heating unit and the edge 10a of the steel strip 10.

Particularly, in this embodiment, inner-side edges 51 a and 52 a of thedepressed portions 51 and 52 of the shielding plate 301 (301 d) aredisposed on the edge 10 a side compared to the maximum current passingregion 56, and outer-side edges 51 b and 52 b of the depressed portions51 and 52 are disposed on the edge side 10 a compared to an edge currentpassing region 57 that is a region through which an eddy current flowingto the vicinity of the edge 10 a of the steel strip 10 passes. Here,among edges of the depressed portions 51 and 52, the inner-side edges 51a and 52 a are edges that are closest to a central portion in the widthdirection of the steel strip 10 and that are closer to the correspondingdepressed portions 52 and 51 (or the central portion of the shieldingplate 301 d in the sheet conveyance direction). In addition, among edgesof the depressed portions 51 and 52, outer-side edges 51 b and 52 b areedges that are farther from the central portion of the steel strip 10 inthe width direction and that are farthest from the correspondingdepressed portions 52 and 51 (or the central portion of the shieldingplate 301 d in the sheet conveyance direction).

In this embodiment, due to the shielding plate 301 disposed as describedabove, a decrease in the temperature of the steel strip 10 in thevicinity of the edge 10 a is suppressed. Hereinafter, a mechanism, whichsuppresses a decrease in temperature of the steel strip 10 in thevicinity of the edge 10 a due to the shielding plate 301, will bedescribed.

As shown in FIG. 9C, when the induction heating unit is operated, mainmagnetic fields 58 a to 58 c are generated, and therefore eddy currents60 a to 60 e flow to an edge side of the steel strip 10 in the sheetwidth direction. In addition, a magnetic field 59 i is generated by theeddy currents 60 a to 60 e. In addition, as shown in FIGS. 9A to 9C,eddy currents 53 to 55 flow through the shielding plate 301 (301 a and301 b). The eddy current 53 is an eddy current flowing along a rhombicedge portion of the shielding plate 301 (main shielding plate 50 a). Onthe other hand, the eddy currents 54 and 55 are currents flowing alongan edge portion of the depressed portions 51 and 52 of the shieldingplate 301. In this manner, in the shielding plate 301, the edge currents53 to 55 flow to the rhombic edge portion of the shielding plate 301 andedge portion of the depressed portions 51 and 52 of the shielding plate301 in a concentrated manner. Furthermore, magnetic fields 59 a to 59 hare generated by the eddy currents 53 to 55.

As a result, as shown in FIG. 9C, a repulsive force is generated betweenthe eddy currents 54 and 55 that flow through the shielding plate 301(301 a and 301 b) and the eddy current 60 that flows through the steelstrip 10. Due to this repulsive force, the eddy current 60 (60 a to 60e) flowing through the edge portion of the steel strip 10 moves to aninner side (in an arrow direction shown under the steel strip 10 in FIG.9C) of the steel strip 10 and a current density in a region in which atemperature decreases in the conventional technique increases.Therefore, a decrease in temperature in the vicinity of the edge (regionslightly to the inside of the edge) of the steel strip 10 may besuppressed, and therefore the shielding plate 301 can adjust the degreeof electromagnetic coupling between a region of the steel strip 10 onthe edge side in the sheet width direction and the heating coils 24 and28. Here, the shielding plate 301 is made of copper, and a necessaryproperty is maintained even at a high temperature. Therefore, even whenthe shielding plate 301 is exposed to high temperatures, a decrease intemperature of the steel strip 10 in the vicinity of the edge thereofcan be suppressed.

Conversely, in a case the depressed portion is not present in theshielding plate 31 like the first embodiment, the eddy currents 53 and54 do not flow through the shielding plate 31 as shown in FIGS. 9A and9C, and an eddy current flows to the rhombic edge portion of theshielding plate 31 in a concentrated manner. Therefore, an eddy currentthat flows to the vicinity of the edge of the steel strip 10 does notreceive a force biased to an inner side (central side) of the steelstrip 10, and a current density of a region (region slightly to theinside of the edge of the steel strip 10) in which a temperaturedecreases does not increase. Therefore, a decrease in temperature in thevicinity of the edge of the steel strip 10 may not be suppressed.

As described above, the inventors found that when the depressed portions51 and 52 are formed in the shielding plate 301 made of copper, and theshielding plate 301 is disposed in such a manner that the depressedportions 51 and 52 are opposite to the vicinity of the edge of the steelstrip 10, a decrease in temperature in the vicinity of the edge of thesteel strip 10 can be suppressed. To confirm this finding, the inventorsmeasured the temperature distribution in the sheet width direction of aconductive sheet (corresponding to the steel strip 10) in a case wherethe shielding plate 301 according to this embodiment is used and in acase where the shielding plate 31 according to the first embodiment isused, respectively.

FIGS. 10A and 10B show views illustrating an example of a temperaturedistribution of a conductive sheet, which is heated by the inductionheating unit, in the sheet width direction.

Specifically, FIG. 10A shows a graph with respect to the inductionheating unit (the induction heating unit according to this embodiment)using the shielding plate 301 according to this embodiment. On the otherhand, FIG. 10B shows a graph with respect to the induction heating unit(the induction heating unit according to the first embodiment) using theshielding plate 31 according to the first embodiment. In addition, thehorizontal axis of graphs shown in FIGS. 10A and 10B indicates aposition in the sheet width direction of the conductive sheet, aposition “0” in the horizontal axis corresponds to an edge of theconductive sheet, and a position “250” corresponds to the center of theconductive sheet. On the other hand, the vertical axis represents anincrease in temperature (temperature increase) of the conductive sheetdue to heating. Here, experimental conditions of graphs shown in FIGS.10A and 10B are as follows.

Width of heating coil: 250 [mm] (length in a sheet conveyance direction)

Core: Ferrite core

Heating material: Non-magnetic SUS (stainless) sheet (a width of 500[mm], and a thickness of 0.3 [mm])

Sheet conveyance speed: 8 [mpm (m/minute)]

Heating temperature: 30 to 130 [° C.] (a temperature increase at acentral portion is set to 100 [° C.])

Frequency of power source: 29 [kHz], 21 [kHz], and 10 [kHz]

Material of shielding plate: Copper

In addition, the closer the relative permeability of a materialapproaches 1, the more easily the temperature in the vicinity of an edgedecreases. In addition, when the temperature of the conductive sheet(material to be heated) is equal to or higher than the Curietemperature, the relative permeability of the conductive sheetbecomes 1. Therefore, the non-magnetic SUS (stainless) sheet was used asthe heating material having the relative permeability of 1.

As shown in FIG. 10A, in the induction heating unit using the shieldingplate 301 according to this embodiment, it can be understood that whenthe frequency is changed in the order of 29 [kHz]→21 [kHz]→10 [kHz], thetemperature of the edge decreases, and a decrease in temperature in thevicinity of the edge (here, at a position of “50” to “100” in thehorizontal axis) is suppressed (the temperature distribution in thesheet width direction becomes uniform).

On the other hand, as shown in FIG. 10B, in the induction heating unitusing the shielding plate 31 according to the first embodiment, it canbe understood that when the frequency is changed in the order of 29[kHz]→21 [kHz]→10 [kHz], the temperature of the edge decreases, but thedecrease in temperature in the vicinity of the edge (here, at a positionof “50” to “100” in the horizontal axis) becomes large.

In addition, in a case where the shielding plate is not provided, thetemperature in the vicinity of the edge (here, at a position of “50” to“100” in the horizontal axis) does not decrease. However, since thetemperature increase in the edge becomes substantially 500 [° C.], theedge was over-heated.

As described above, according to this embodiment, the depressed portions51 and 52 are formed in the shielding plate 301 made of copper, theshielding plate 301 is disposed between the upper and lower side heatingcoils 24 and 28 and the steel strip 10 in such a manner that thedepressed portions 51 and 52 face the vicinity of the edge of the steelstrip 10. Therefore, even when the steel strip 10 is exposed to hightemperatures, a decrease in temperature of the steel strip 10 in thevicinity of the edge thereof can be suppressed.

Furthermore, in the induction heating system provided with the controlunit 100 and the induction heating unit having the shielding plate 301,even when the sheet conveyance speed varies, since the frequency of theAC power does not vary, it is not necessary to consider a variation(temporal variation) of the eddy current that is generated in the edgeportion of the steel strip 10. Therefore, when the control unit 100 isused in the induction heating system, even when operational conditionsvary, a temperature increase in the vicinity of the edge can beappropriately controlled by the shielding plate 301. Furthermore, sincethe depressed portions 51 and 52 are formed in the shielding plate 301,even when the relative permeability varies in response to a heated stateof the steel sheet, the temperature distribution in the vicinity of theedge can be appropriately controlled due to the depressed portions 51and 52. Therefore, in the configuration according to this embodiment, itis possible to cope with a change in heating speed in a relativelyflexible manner.

In addition, in the above-described embodiments (the first embodiment tothe third embodiment), the shielding plates 31 and 301 are not limitedto a plate made of copper. That is, the shielding plates 31 and 301 maybe formed by any material as long as this material is a conductor havinga relative permeability of 1 (for example, metal that is a paramagneticsubstance or a diamagnetic substance). For example, the shielding plate31 may be formed of aluminum.

In addition, in this embodiment, the positional relationship between thesteel strip 10 and the shielding plate 301 is not particularly limitedas long as the depressed portions of the shielding plate 301 and thesteel strip 10 (also including a plane extended from the steel strip 10)are opposite to each other in a region that is present on the edge 10 aside compared to the maximum current passing region 56. However, it ispreferable that a region between the maximum current passing region 56and the edge 10 a of the steel strip 10, and at least a part of thedepressed portions of the shielding plate be opposite to each other asshown in FIG. 9B in order for a repulsive force to be reliably generatedbetween the eddy current flowing through the shielding plate 301 and theeddy current flowing through the steel strip 10.

In addition, in this embodiment, a description has been made withrespect to a case in which the two depressed portions are formed in theshielding plate as an example, but the number of the depressed portionformed in the shielding plate is not limited.

In addition, in this embodiment, an illustration has been made withrespect to a case in which the shape of the depressed portions 51 and 52is a rhombic shape as an example. However, the shape of the depressedportions 51 and 52 may be any shape as long as the eddy current may bemade to flow through the steel strip 10 along the edge portion of thedepressed portions 51 and 52. The shape of the depressed portions 51 and52 may be, for example, an ellipse, a rectangle other than a rhombicshape, or other square shapes. At this time, when a depressed portion inwhich the length in the sheet conveyance direction is longer than thatin a direction orthogonal to the sheet conveyance direction is formed,the eddy current can be easily made to flow along an edge portion of thedepressed portion. Therefore, it is preferable to form a depressedportion in which the length in the sheet conveyance direction is longerthan that in the direction orthogonal to the sheet conveyance direction.In addition, the shape of the depressed portion in the shielding plateis not necessary to have a closed shape. For example, the depressedportion may be formed in an end portion of the shielding plate.

Furthermore, copper is normally used for the upper side heating coil 24and the lower side heating coil 28, but a conductor (metal) other thancopper may be used. In addition, an induction heating system other thanthe continuous annealing line may be adopted. In addition, thedimensions of the cores 23 and 27 shown in FIG. 2A may be appropriatelydetermined within a range in which the cores 23 and 27 are notmagnetically saturated. Here, the generation of magnetic saturation inthe cores 23 and 27 may be determined from magnetic field strength [A/m]that is calculated from the current flowing through the heating coils 24and 28.

In addition, in the above-described embodiments, both of the upper sideinductor 21 and the lower side inductor 22 are provided as an example,but either the upper side inductor 21 or the lower side inductor 22 maybe provided. Furthermore, the size of the gap is not particularlylimited.

In addition, all of the above-described embodiments of the presentinvention illustrate only a specific example for executing the presentinvention, and a technical scope of the present invention is not limitedto the embodiments. That is, the present invention may be executed withvarious forms without departing from the technical scope or criticalfeatures thereof.

INDUSTRIAL APPLICABILITY

It is possible to provide a control unit of an induction heating unit,an induction heating system, and a control method of the inductionheating unit, in which a temperature distribution in the sheet widthdirection of a conductive sheet is made more uniform compared to that inthe conventional techniques, even when the sheet conveyance speed of theconductive sheet varies in a case where the conductive sheet is heatedusing a transverse type induction heating unit.

REFERENCE SYMBOL LIST

10: Steel strip (Conductive sheet)

20: Induction heating unit

23, 27: Core (Magnetic core)

24: Upper side heating coil (Heating coil)

28: Lower side heating coil (Heating coil)

31 a to 31 d: Shielding plate

51, 52: Depressed portion (Valley portion)

100, 200: Control unit of induction heating unit

110: Rectifying unit

120: Reactor

130: Magnetic energy recovery switch (MERS)

131 to 134: First to fourth reverse conductivity type semiconductorswitches

140: Gate control unit

150: Output current setting unit

160: AC power supply

170: Current transformer (Current measuring unit)

180: Frequency setting unit

210: Output transformer

301: Shielding plate

S1 to S4: Semiconductor switches

D1 to D4: Diodes

What is claimed is:
 1. An induction heating system which allows analternating magnetic field to intersect a sheet surface of a conductivesheet which is being conveyed to inductively heat the conductive sheet,the induction heating system comprising: an induction heating unitincluding a heating coil which is disposed to face the sheet surface ofthe conductive sheet; and a control unit which controls an AC power tooutput to the heating coil, wherein the induction heating unit includes:a core around which the heating coil is wound; and a shielding platewhich is disposed to face a region including an edge of the conductivesheet in a width direction, the shielding plate having a depressedportion at a surface which faces the conductive sheet, and the shieldingplate being formed from a metal that is a paramagnetic substance or adiamagnetic substance, and wherein the control unit includes: a magneticenergy recovery switch which outputs the AC power to the heating coil; afrequency setting unit which sets an output frequency in response to atleast one of a relative permeability, a resistivity, and a sheetthickness of the conductive sheet; and a gate control unit whichcontrols a switching operation of the magnetic energy recovery switch onthe basis of the output frequency set by the frequency setting unit. 2.The induction heating system according to claim 1, wherein the shieldingplate is disposed in such a manner that a region, which is closer to theedge of the conductive sheet than a region in which an eddy currentflowing to the conductive sheet becomes a maximum, and the depressedportion face each other.
 3. The induction heating system according toclaim 1, wherein an inner-side edge out of edges of the depressedportion, which is on a closer side of a central portion in the widthdirection of the conductive sheet, is disposed in such a manner that theedge of the conductive sheet is closer to the inner-side edge than aregion in which the eddy current flowing to the conductive sheet becomesa maximum, and an outer-side edge out of the edges of the depressedportion, which is on a farther side of the central portion in the widthdirection of the conductive sheet, is disposed in such a manner that theedge of the conductive sheet is closer to the outer-side edge than aregion through which the eddy current flows to the edge of theconductive sheet.
 4. The induction heating system according to claim 1,wherein the frequency setting unit acquires an attribute informationwhich specifies the relative permeability, the resistivity, and thesheet thickness of the conductive sheet, and selects a frequencycorresponding to the acquired attribute information as the outputfrequency with reference to a table in which the relative permeability,the resistivity, and the sheet thickness of the conductive sheet, andthe frequency are correlated with each other and are registered inadvance.
 5. The induction heating system according to claim 1, whereinthe control unit further comprises: an output current setting unit whichsets an output current value in response to at least one of the relativepermeability, the resistivity, and the sheet thickness of the conductivesheet; a current measuring unit which measures an alternating currentwhich flows through the induction heating unit; and a power supply unitwhich supplies a DC power to the magnetic energy recovery switch andadjusts the alternating current which is measured by the currentmeasuring unit to the output current value which is set by the outputcurrent setting unit, wherein the magnetic energy recovery switch issupplied with the DC power by the power supply unit and outputs the ACpower to the heating coil.
 6. The induction heating system according toclaim 5, wherein the output current setting unit acquires an attributeinformation which specifies the relative permeability, the resistivity,and the sheet thickness of the conductive sheet, and selects a currentvalue corresponding to the acquired attribute information as the outputcurrent value with reference to a table in which the relativepermeability, the resistivity, and the sheet thickness of the conductivesheet, and the current value are correlated with each other and areregistered in advance.
 7. The induction heating system according toclaim 1, further comprising: an output transformer which is disposedbetween the magnetic energy recovery switch and the induction heatingunit, lowers an AC voltage which is output from the magnetic energyrecovery switch, and outputs the lowered AC voltage to the heating coil.8. The induction heating system according to claim 5, wherein themagnetic energy recovery switch includes: first and second AC terminalswhich are connected to one end and an other end of the heating coil,respectively; first and second DC terminals which are connected to anoutput terminal of the power supply unit; a first reverse conductivitytype semiconductor switch which is connected between the first ACterminal and the first DC terminal; a second reverse conductivity typesemiconductor switch which is connected between the first AC terminaland the second DC terminal; a third reverse conductivity typesemiconductor switch which is connected between the second AC terminaland the second DC terminal; a fourth reverse conductivity typesemiconductor switch which is connected between the second AC terminaland the first DC terminal; and a capacitor which is connected betweenthe first and second DC terminals, wherein the first reverseconductivity type semiconductor switch and the fourth reverseconductivity type semiconductor switch are connected in series in such amanner that conduction directions at the time of a switch-off becomeopposite to each other, the second reverse conductivity typesemiconductor switch and the third reverse conductivity typesemiconductor switch are connected in series in such a manner thatconduction directions at the time of the switch-off become opposite toeach other, the first reverse conductivity type semiconductor switch andthe third reverse conductivity type semiconductor switch have the sameconduction direction at the time of the switch-off as each other, thesecond reverse conductivity type semiconductor switch and the fourthreverse conductivity type semiconductor switch have the same conductiondirection at the time of the switch-off as each other, and the gatecontrol unit controls a switching operation time of the first and thirdreverse conductivity type semiconductor switches and a switchingoperation time of the second and fourth reverse conductivity typesemiconductor switches on the basis of the output frequency which is setby the frequency setting unit.